Solar cells made from cheap nanocrystal-based inks have the
potential to be as efficient as the conventional inorganic cells
currently used in solar panels, but can be printed less expensively. Solexant,
a company in San Jose, CA, is currently manufacturing solar cells to
test the technology. In order to compete with other thin-film solar
companies, Solexant is banking on simpler, cheaper printing processes
and materials, as well as lower initial capital costs to build its
plants. The company expects to sell modules for $1 per watt, with
efficiencies above 10 percent.

Nanocrystal solar: The solar cells at top were made on a
roll-to-roll printer from an ink consisting of the rod-shaped inorganic
semiconducting nanocrystals shown below. The cells were printed on a
flexible metal foil and will be topped with a glass plate.
Credit: Solexant

The company has licensed methods for growing nanocrystals and making them into inks from Paul Alivisatos, professor of nanotechnology at the University of California, Berkeley and interim director of the Lawrence Berkeley National Laboratory.
(Alivisatos is on Solexant's board of directors.) Alivisatos says the
advantage of these materials is their potential to combine low cost
with high performance. Solar cells made from crystalline silicon are
efficient at converting sunlight into electricity, but they're
expensive to manufacture. To bring down the cost, companies have been
developing thin-film solar cells from semiconductors that don't match
crystalline silicon's performance but are much less expensive to make.

Solexant's goal is to make cheap thin-film solar cells with
relatively high efficiencies. It would not disclose what the
nanoparticle inks are made of, but the company says they are
suspensions of rod-shaped, semiconducting nanocrystals that are four
nanometers in diameter and 20 to 30 nanometers long. The Solexant cells
are printed on a metal foil as the substrate. Nanocrystal films are
simple to print but have poor electrical properties. Electrons tend to
get trapped between the small particles. "The trick with these cells is
how to deposit the materials on the fly in a way that makes a very
conductive surface," which in turn ensures decent light-to-electricity
conversion, says Alivisatos. Solexant begins with nanocrystals because
they're easier to print, and heats them as they're printed, causing
them to fuse together into larger, high-quality microcrystals that
don't have as many places for electrons to lose their way.